/* 类选择器 */
.color {
    color: red;
}

/*匹配既是p标签，又有color类的元素*/
p.color {
    color: blue;
}

.box_red {
    background-color: red;
    width: 20px;
    height: 20px;
}
.box_blue {
    background-color: blue;
    width: 30px;
    height: 10px;
}
/*匹配同时含有box_red box_blue类的元素*/
.box_red.box_blue {
    background-color: purple;
    width: 10px;
    height: 30px;
}
